Method of determining the fluid content of well cores



3m., Ml@ M. MUSKAT ET A1. 29%@93 METHDVOF DETERMINING THE FLUID CONTENT OF WELL CORES Filed Feb. 19, 1945 y grumvos- MORRIS MUSKAT NORMAN D. coe GESHALL Patented Jan. 4, 1949 UNITED STATES PATENT( QFNFIC 2,458,093 :jfl METHOD F DETERMINING THE FLUID ff CONTENT oF WELL ooREs .i i

Morris Muskat, Oakmont, and Norman DQ, Cog-T geshall, OHara rlownship, Allegheny County;`

Pa., assignors to Gulf Research &Developnie`nt' Compa-ny, Pittsburgh, Pa., a corporation of Delaware Application February 19, 1945, Serial No. 578,748

1 Claim. (Cl. Z50-83) This invention concerns an improved method of determining the amount of fluid in a core obtained from a certain level in a well bore. More particularly, it involves a method of determining by means of a radioactive technique the drilling fluid contentof a well core, so that this information may be used as a method of logging or applied as a correction to connate fluid determinations.

In the drilling and producing of oil wells, it is customary to cut cores of the formations pene` trated.` These cores are useful for identifying formations which might be oil bearing, and they are particularly useful in determining the productivity which might be expected of the various formation horizons encountered in the well.

In the determination of the productivity of an oil bearing horizon, a core sample is subjected to certain well known tests. These tests involve determination of porosity and permeability by known methods. It is also necessary to know the pore content of the producing formation. This is done by taking bottom hole samples under pressure, bringing them to the surface and analyzing them. The samples may contain gas or oil or connate water.

A serious diiliculty in the above determinations arises from the fact that Wells are drilled with the use of a iiuid of some type to cool the drill and remove drill cuttings and carry them to the surface. This drilling fluid is commonly maintained with a bottom hole pressure slightly greater than that of the formation being drilled, in order that the well may at all times be kept under control. Any formation which is porous enough to produce oil Will obviously have forced into it a certain amount of the drilling uid during the drilling process and this iiuid contaminates the connate uid in the formation to such an extent that a subsequent test on a sample brought to the surface may not give reliable results. In case an oil base drilling fluid is used, the oil content analysis may obviously be in error. If an aqueous drilling uid is used, the connate water analysis may be in error.

A further limitation arises from the fact that the determination of connate uids is a time consuming and expensive laboratory procedure and is therefore ordinarily not used except `for relatively widely spaced samples. A more continuous log ofthe borehole with respect to the amount of drilling fluid entrained in the core has heretoforenot been commercially practicable because of the difficulty and uncertainty heretofore incident to the measurements. The method of our invention when applied to cores would permit such measurements to be-made very simply and quickly so that one may vlog the cores with respect to the amount ofdrilling fluid invasion as a basis of correlation andindexing the type` of strata which the drill has traversed. Such a log will reect the variations in permeability and porosity of the strata.` An especially advantageous feature of our invention isthat quantitative measurements of the drilling fluid invasion maybe made before and without the tedious process of extraction, whereas in other methods extraction is necessary. 1^ `f l One of the objects oi` this invention is the eval# uation of a correction in a core fluid determination for the arnountof `drilling iiuid foreign to the core which was forced into the core during the drilling process beforeor during the cutting of the core. l

Another object of this invention isto provide a simple and accurate methodof correcting a' core fluid determination for -the amount of `iiuid absorbed by the core from thedrilling fluid.

A Istillfurther object of thislinventionis to determine the amount of drilling fluid absorbed by the core by means of introducing a radioactive tracer into the fluid. l f

An ancillary objectlof this invention is'topro* vide a simple, quick and accurate method of log-1 ging the cores ofa borehole *with respect to drill` ing fluid invasion." l l Y l These and other useful objects of the inventior. are accomplished in the manner described in the following specification;` l l l A fuller understanding of our `invention `may be obtained byreference to the drawing, in which:

Figure 1 shows inlsection` a 'coresample taken from a Well bore in the manner of our invention;

Figure l2 showsv an arrangement of measuring device for determining the` amount :of 'drilling fluidpenetrating such a core sample; and

Figure 3 shows another arrangement of measuring device for determining the amount `of drilling `fluid in such a core sample;y

The invention comprises in its simplest form adding to the drilling `mudv in known and controlled proportions a radioactive material'vvhich becomes homogeneously mixed in the mud. The core is cut with the use of 'this radioactive mud, and then later the radioactivity `in the coresame ple is measured. The measured radioactivity will allow a determination of the amount of drilling fluid which "actually l. penetrated into `the core'. This information may `then'be l'plotted inthe form of a well log. If the core is extracted to determine connate fluid, the information obtained by our 3 invention may be used to identify that part of the extracted fluid which is entrained drilling fluid.

Several types of radioactive materials are advantageously added to the drilling fluid. Naturally radioactive material in the form of a salt of uranium, thorium or radium may be used. If the drilling fluid is of the aqueous type, the radioactive salts should be water soluble. In the case of oil base drilling fluids, they .should be oilsoluble such as, for instance, soaps of radioactive elements. `Radium naphthenate or barium-radiumpetronate are satisfactory oil ,soluble tracers. It is also possible to use artificially rad-reactivated substances. Examples of `such artificial radioactive tracer materials are the isotopes of ordinary sodium, namely Na22 or ,Na24 which may be In Figure 1 We have shown in cross section a sample of core 2 'cut from a well using mud containing a dissolved radioactive material. The core is somewhat porous 'and a .certain .amount of radioactive drilling fluid penetrates the core as indicated in the iigures by numeral 3. While the entire core 2 .contains connate 4fiuid an extraction would also include the extraneous fluid 3. The amount of fluid 3 is some function of the permeability and porosity of core V2. By means of this invention one may .determine the amount 'of fluid 53. This may then be subtracted from the total amount of extracted `fluid Ain the core to arrive at the correct 4amount of connate u-id.

After v,the core yhas been cut and removed to the surface, its radioactivity 4may be determined by means of the apparatus shown -inFigure 2. Numeral I represents a section of core removed from the core barrel, the entrained radioactive drilling fluid being indicated by numeral 3. A small hole II may be drilled into the face of the core `and into Athis vhole is inserted a detector of radioactivity, .such as an ionization chamber or Geiger counter indicated by numeral I2. The counter I2 kis supplied high potential by the battery I3 through resistance I4 'and leads I5. The potential across :resist-or I4 may be amplied by amplier .I6 Whose output is indicated on meter 'I'I. The meter lI'I gmay be calibrated to read directly `the amount :of entrained radioactive drilling uid indicated inthe core-sample vI II.

Another method oi determining the radioactivity of the core is lby the use .of a vspecially shaped ionization chamber shown in Figure 3. Numeral 2| represents A.the .core being studied having entrained radioactive drilling vii-uid 3. Numeral 22 represents -a vessel whichcontains an ionization chamber. Stopcock 23 is a-connection to 'the air -or to a suitable gas reservoir. The vessel 212 contains three cylindrical conductors, 24 being the outermost, 25 being the intermediate one and 26the innermost one. All are concentric andinside `theannular vessel '22. Leads are brought .out asfindicatedat points 3l and con- 4 nected as shown to battery 21 and resistor 28. When the core sample is placed inside this annular vessel, the gamma rays emitted from the core pass through ythe vessel and electrodes and ionize the gas .contained'in the vessel. As a result of the potential applied bythe battery 2l, all the ions formed in the region between 24 and 25 and between 25 and 26 are collected. The resulting current traverses resistance 28 and the consequent voltage drop across resistance 28 is a measure of the radioactivity of the core. The potential drop across 23 is amplified by the amplifier 29 and the outputfed to meter 30. The indicating meter 30 therefore gives a reading proportional to the ionization current which in turn is a measure of the drilling fluid absorbed by the core. This device is easily calibrated by merely inserting, instead of core 2l, a thin walled container filled with a measured sample of drilling fluid. The apparatus of Figure 3 is particularly'adapted to logging of long cores in which case the annular vessel 22 may simply be slipped over the core and measurement made along the. length of the core.

The concentration of the radioactive material used'in the drilling iiuid need not be very large because the natural radioactivity of the rocks normally encountered in oil producing formations is very small. The determinationv may in any case be improved by subtracting from the reading of meters II or 30 any indication obtained from a core not drilled with radioactive mud. Such core samples are readily available, so that the reading obtained with a blank core would .be known.

In the practice of the invention, `the radioactive material is preferably contained in the uid in a dissolved state, so that it will not be filtered by small pores in the core sample. It should, furthermore, be of such .chemical composition that it will not react with or be precipitated by the rocks or the fluids encountered in drilling, We have mentioned several radioactive compounds which may be used in our invention and these are by way of illustration only, many other similar compounds being available for .accomplishing the objects of our invention.

What We claimas our invention is:

A method of determining the connate fluid content of a well core sample comprising adding to the `drilling fluid a known concentration of artificially radioactivated material soluble in the drilling iluid, cutting and removing the core sainple from the well, measuring the radioactivity of a known quantity of core sample to determine the amount of drilling iluid absorbed in the sample, determining lthe nature and quanti-ty of total fluids in the core sample and subtracting therefrom the said determined ,amount of drilling fluid absorbed.v

MORRIS MUSKA'I. NORMAN D. COGGESHALL.

REFERENCES errno The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,330,829 Lundberg Oct. 5, 1943 2,349,712 Fearon May 23, 1944 2,352,993 Albertson July 4, 1,944

2,364,975 Heigl Dec. 12, 1944 

